At the outset, assuming that they are strangers to each other, the only thing commonly known between the speaker/writer and the listener/reader is language—how it works and the meanings that arise from the various combinations of its words. There is therefore no way for the listener/reader to know precisely what’s in the speaker’s/writer’s mind—in particular, whether a noun used is meant to be definite or indefinite—until he or she has written the statement to begin with.
Now, when you tweet “I visited THE hot shopping spots in South Korea,” the reader on Twitter obviously will perceive you as someone who wants to make the impression of being extremely knowledgeable about the hot shopping spots in South Korea, having presumably visited all or most of them. It makes no difference whether your reader knows everything or doesn’t know anything about those hot shopping spots, or whether you are just making a tall claim to begin with. And there need not be any confusion or guessing about that declaration of yours, because it’s not as if you made that statement ex cathedra—not subject to clarification or challenge like a papal edict. In fact, if the Twitter reader (who just might happen to be a well-informed Seoul resident) is interested at all in your tweet, he or she will likely tweet back: “Precisely which shopping spots are those? And how do you know they are ‘hot’?” Then a real conversation starts on Twitter wherein you’ll be obliged to support your contention that you have indeed “visited THE hot shopping spots in South Korea.” Since it’s highly improbable that you’ve visited all or most of those ‘hot’ shopping spots, you’d likely be forced to scale down your claim to, say, “Well, I actually visited only four of them in Seoul and I thought they were ‘hot’ because there were so many people in them…” Then you may get a follow-up tweet like, say, “Which four did you actually visit? And how big was the crowd in each of them during your visit?” And the Twitter conversation continues until the reader and you have clarified and exhausted the subject to your mutual satisfaction.
My point in the rather elaborate explanation above, English Maiden, is that you really shouldn’t put too much store in a single sentence or two as the be-all and end-all of communicating an idea—which I’m afraid is the communication culture that Twitter inadvertently promotes. Because of the 140-word limit to each tweet, the tweeter is constrained to give an unwarranted sense of factuality or finality to every tweet he or she makes. In a real-world conversation, however, everything said is at best tentative, subject to being qualified or modified for greater accuracy in the course of the conversation. There’s much less pressure for the speaker to overstate or exaggerate with a strong declarative statement like “I visited THE hot shopping spots in South Korea”; more likely, the norm for that opening statement will be the more unprepossessing and modest “I visited hot shopping spots in South Korea”—without the “the.” After all, there will be lots of room to qualify the statement to a level of accuracy closer to the truth. This level of qualification, however, is something that’s difficult to achieve in a medium like Twitter, which I think is much more suited to brief announcements rather than to discussions of complex ideas.
This being the case, for modesty’s sake as well as for semantic correctness, I think it would be more appropriate not to use the article “the” at all in that sample tweet of yours. A bare-bones “I visited hot shopping spots in South Korea” or a qualified “I visited a few hot shopping spots in South Korea” definitely will be much more advisable.
